Alternating current networks and devices are typically designed to operate on sinusoidal 50 Hz or 60 Hz alternating voltages and currents. The voltage and current waveforms in modern power distribution systems are seldom sinusoidal, however, because of the increased use of nonlinear electrical loads such as variable speed motor drives. Such nonlinear loads draw a nonsinusoidal current from a sinusoidal voltage source. This results in a waveform distortion which propagates outward from the nonlinear electrical load into the rest of the network. The distorted waveform typically includes frequency components which are multiples of the original frequency of the electrical power (50 Hz or 60 Hz). These high frequency components are called harmonics. The harmonics are created by the nonlinear electrical load. In 60 Hz power distribution systems, harmonics at 300HZ, 420 Hz, 660 Hz and other odd-multiples of frequency are particularly significant. It is desirable to limit the production and propagation of harmonics because harmonics can disrupt and damage sensitive electrical devices drawing power from the power distribution system.
One problem with limiting the production of harmonics is that they are difficult to trace. Their source cannot easily be found and measured. Nearly all electric customers create harmonic pollution which is spread by the distribution system to other customers. Other customers may experience problems due to the harmonics, but the source of the offending harmonics is difficult to identify. If harmonic current sources could be located and accurately measured, harmonic pollution could be more effectively eliminated from power distribution systems.
Two methods have been proposed for the purpose of limiting the amount of harmonic pollution present in a power distribution system. One method involves the establishment of limits on the amount of harmonic energy generated by customers and utilities. Power distribution systems are designed to operate within these limits. This scheme has been widely adopted in the power industry at present. Reference can be made to The IEEE Standard 519, "IEEE Recommended Practice for Harmonic Control" concerning the limits on harmonic energy generation. A major problem with this scheme is that if the limits are exceeded by a customer, the only enforcement power the utility has is to disconnect the customer, which is not desirable. As a result, an incentive method has recently been proposed in which harmonic generators are charged an amount commensurate with their harmonic pollution level when the limits are exceeded. Various methods have been proposed to measure the harmonic pollution generated by customers in order to implement the incentive method. Reference can be made to U.S. Pat Nos. 5,307,009, 5,302,890, 5,298,859, 5,298,856, 5,298,855, 5,298,854, and 5,212,441 concerning methods of measuring harmonic pollution levels and defining an incentive rate pricing structure. The methods described in these patents, however, generally do not differentiate sources of harmonic pollution connected to the same power lines. Also, these methods do not compensate for circuit changes which may affect other pollution sources. For example, an impedance change in one pollution source will affect the amount of harmonic pollution emitted by other pollution sources. Therefore, a change in one pollution source may be mistakenly attributed to other pollution sources.
The successful implementation of a rational and equitable incentive method for limiting harmonic pollution requires the accurate and quantitative measurement of individual harmonic pollution sources connected to a common power line. Harmonic pollution contributions from a power utility and power customer must be distinguished. Also, the impact of impedance changes on the limit violation must be quantified and compensated for. For example, changes in the impedance of the utility can result in changes in the amount of harmonic energy emitted by a customer. In such cases, the customer should not be held responsible for the resulting changes in harmonic pollution emitted because the customer has no control over the harmonic impedance of the utility. These problems are not solved by known techniques which essentially assume that the electric utility does not produce harmonic pollution, i.e. that the customer is entirely responsible for all the harmonic pollution at the connection point between the utility and customer.
There are many commercially available instruments for measuring parameters of harmonic disturbances such as the magnitudes of the harmonic currents and voltages. Some such instruments measure the direction of harmonic energy flow, thereby enabling the harmonic pollution source to be located. If the energy flows from the left to the right of the metering point, the harmonic power source is on the left, and vice versa. U.S. Pat Nos. 4,667,152 to Hayes et al., and 5,508,623 to Heydt et al. operate according to this principle. The harmonic energy-flow technique, however, has the following major shortcomings:
1) It only detects the net direction of harmonic energy flow. The method cannot determine if there are harmonic current sources on both sides of the metering point.
2) The method is qualitative. It is unable to measure the relative contributions of different harmonic current sources if the sources are located on opposite sides of the metering point.
3) The method detects direction based on net harmonic power. Intervening impedances can thus result in misleading information about the locations of the harmonic current sources. For example, it is possible for a right side harmonic current source to produce more harmonic energy than a left side harmonic current source, but for the net harmonic energy flow to be from left to right.
What is needed is a technique for measuring parameters of a single harmonic pollution source, where the measurement is not affected by other harmonic pollution sources or impedances connected to the same power distribution system. The techniques should be able to compensate for impedance changes or harmonic energy changes external to the harmonic pollution source of interest. More specifically, the method should be able to individually measure the electric utility contribution and customer contribution to the total harmonic pollution present in the power distribution system.